Upstream Pilot Structure In Point To Multipoint Orthogonal Frequency Division Multiplexing Communication System

ABSTRACT

A central access network unit comprising a processor configured to assign a plurality of upstream training blocks from an upstream OFDM symbol to a plurality of downstream network units, wherein the OFDM symbol comprises a plurality of pilot subcarriers equally spaced across an upstream RF spectrum in a pre-determined time interval, and wherein each upstream training block comprises a different subset of the pilot subcarriers that are non-consecutive and situated across the upstream RF spectrum, and generate one or more messages comprising assignments of the upstream training blocks, and a transmitter coupled to the processor and configured to transmit the messages to the plurality of downstream network units via a network, wherein the messages instruct at least one of the plurality of downstream network units to transmit a modulated pre-determined sequence at the pilot subcarriers corresponding to the upstream training block assigned to the downstream network unit.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to U.S. Provisional PatentApplication 61/776,488, filed Mar. 11, 2013 by Xiaofeng Zhang, et. al.,and entitled “Upstream Pilot Structure In Point To Multipoint OrthogonalFrequency Division Multiplexing Communication System”, which isincorporated herein by reference as if reproduced in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO A MICROFICHE APPENDIX

Not applicable.

BACKGROUND

A passive optical network (PON) is one system for providing networkaccess over the last mile. PON may be a point-to-multipoint (P2MP)network with passive splitters positioned in an optical distributionnetwork (ODN) to enable a single feeding fiber from a central office toserve multiple customer premises. PON may employ different wavelengthsfor upstream and downstream transmissions. Ethernet passive opticalnetwork (EPON) is a PON standard developed by the Institute ofElectrical and Electronics Engineers (IEEE) and specified in IEEEdocuments 802.3ah and 802.3av, both of which are incorporated herein byreference. Hybrid access networks employing both EPON and other networktypes have attracted growing attention.

SUMMARY

In one embodiment, the disclosure includes a central access network unitcomprising a processor configured to assign a plurality of upstreamtraining blocks from an upstream orthogonal frequency divisionmultiplexing (OFDM) symbol to a plurality of downstream network units,wherein the OFDM symbol comprises a plurality of pilot subcarriersequally spaced across an upstream radio frequency (RF) spectrum in apre-determined time interval, and wherein each upstream training blockcomprises a different subset of the pilot subcarriers that arenon-consecutive and situated across the upstream RF spectrum, andgenerate one or more messages comprising assignments of the upstreamtraining blocks, and a transmitter coupled to the processor andconfigured to transmit the messages to the plurality of downstreamnetwork units via a network, wherein the messages instruct at least oneof the plurality of downstream network units to transmit a modulatedpre-determined sequence at the pilot subcarriers corresponding to theupstream training block assigned to the downstream network unit.

In another embodiment, the disclosure includes a method implemented by aCable Modem Termination System (CMTS) comprising allocating a probingsymbol within a probing frame, wherein the probing frame comprises avariable K number of contiguous probing symbols, and wherein eachprobing symbol comprises a plurality of subcarriers equally spacedacross an upstream frequency spectrum of a data over cable serviceinterface specification (DOCSIS) network in a pre-determined timeinterval, defining a probing pattern in the allocated probing symbol,wherein the probing pattern comprises a set of pilots from scatteredsubcarriers of the allocated probing symbol, and instructing a CableModem (CM) to transmit a probing sequence in the allocated probingsymbol according to the defined probing pattern.

In another embodiment, the disclosure includes a method implemented by aCoaxial Line Terminal (CLT) comprising allocating a specific probingsymbol to a Coaxial Network Unit (CNU) within a probing frame forupstream wideband probing, wherein the probing symbol comprises aplurality of pilots equally spaced across an upstream spectrum of anEthernet passive optical network over coax (EPoC) in a pre-determinedtime interval, allocating a subset of scattered pilots within theprobing symbol to the CNU, receiving the probing symbol from the CNU,performing upstream channel estimation from the received probing symbol.

In yet another embodiment, the disclosure includes a network unit in acoaxial network comprising a receiver configured to receive a messageindicating an assigned upstream training block in an upstream OFDMsymbol comprising a plurality of pilot subcarriers equally spaced in afrequency spectrum of the network unit in a pre-determined timeinterval, wherein the upstream training block comprises a subset of thepilot subcarriers that are non-consecutive and situated across thefrequency spectrum, a processor coupled to the receiver and configuredto generate the upstream training block by modulating a pre-determinedsequence onto the pilot subcarriers of the upstream training block, anda transmitter coupled to the processor and configured to send theupstream training block via the coaxial network.

These and other features will be more clearly understood from thefollowing detailed description taken in conjunction with theaccompanying drawings and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure, reference is nowmade to the following brief description, taken in connection with theaccompanying drawings and detailed description, wherein like referencenumerals represent like parts.

FIG. 1 is a schematic diagram of an embodiment of a unifiedoptical-coaxial network.

FIG. 2 is a schematic diagram of an embodiment of a DOCSIS network.

FIG. 3 is a schematic diagram of an embodiment of a network element(NE), which may act as a node in an EPoC network and/or a DOCSISnetwork.

FIG. 4 is a schematic diagram of an embodiment of a probing symbolcomprising one upstream training block.

FIG. 5 is a schematic diagram of another embodiment of a probing symbolcomprising one upstream training block.

FIG. 6 is a schematic diagram of another embodiment of a probing symbolcomprising three upstream training blocks.

FIG. 7 is a flowchart of an embodiment of an upstream training method.

FIG. 8 is a flowchart of another embodiment of an upstream trainingmethod.

FIG. 9 is a schematic diagram of an embodiment of an upstream trainingmessage encoding.

FIG. 10 illustrates a graph of an embodiment of upstream Signal-to-NoiseRatio (SNR) loss as a function of number of probing downstream networkunits in a single probing symbol.

DETAILED DESCRIPTION

It should be understood at the outset that, although an illustrativeimplementation of one or more embodiments are provided below, thedisclosed systems and/or methods may be implemented using any number oftechniques, whether currently known or in existence. The disclosureshould in no way be limited to the illustrative implementations,drawings, and techniques illustrated below, including the exemplarydesigns and implementations illustrated and described herein, but may bemodified within the scope of the appended claims along with their fullscope of equivalents.

Some hybrid access networks may combine optical networks with coaxial(coax) networks. Ethernet over Coax (EoC) may be a generic name used todescribe all technologies that transmit Ethernet frames over a coaxialnetwork. Examples of EoC technologies may include EPoC, DOCSIS,multimedia over coax alliance (MoCA), G.hn (a common name for a homenetwork technology family of standards developed under the InternationalTelecommunication Union (ITU) and promoted by the HomeGrid Forum), homephoneline networking alliance (HPNA), and home plug audio/visual (A/V).EoC technologies may have been adapted to run outdoor coax access froman Optical Network Unit (ONU) to an EoC head end with connected customerpremises equipment (CPEs) located in subscriber homes. In a coaxialnetwork, physical layer transmission may employ OFDM to encode digitaldata onto multiple carrier frequencies. Some advantages of OFDMtransmission may include high spectral efficiency and robusttransmission (e.g. attenuation at high frequencies in long coaxialwires, narrow band interferers, frequency selective noise, etc.).

An EPoC system may be a hybrid access network employing both optical andcoaxial technologies. The EPoC may comprise an optical segment that maycomprise a PON, and a coaxial segment that may comprise a coaxial cablenetwork. In the PON segment, an OLT may be positioned in a localexchange or central office where the OLT may connect the EPoC accessnetwork to an Internet Protocol (IP), Synchronous Optical Network(SONET), and/or Asynchronous Transfer Mode (ATM) backbone. In thecoaxial segment, CNUs may be positioned at end-user locations, and eachCNU may serve a plurality (e.g. three to four) of end users which may beknown as subscribers. A Fiber Coaxial Unit (FCU) may merge the interfacebetween the PON segment and the coaxial segment of the network. The FCUmay be a single box unit that may be located where an ONU and a CLT arefused together, for example, at a curb or at a basement of an apartmentbuilding. The CLT or FCU may employ OFDM transmission at a physicallayer to communicate with the CNUs.

A DOCSIS network may operate over a hybrid fiber coax (HFC) network andmay be structurally similar to an EPoC network. The DOCSIS network maycomprise a CMTS positioned in a local exchange or central office wherethe CMTS may connect the HFC network to a backbone network. The CMTS mayserve a plurality of CMs positioned at end-user locations. In someembodiments, a CMTS may be integrated with P2MP OFDM communicationfunctionalities (e.g. channel estimation, scheduling).

In OFDM communication, a physical layer channel may be established priorto data transmission, for example, by performing channel training and/orestimation. In an embodiment, a CLT may designate an upstream OFDMsymbol (e.g. probing symbol) for upstream channel measurement (e.g.upstream probing). The probing symbol may span in time and frequency,for example, the probing symbol may comprise a plurality of subcarriers(e.g. pilot subcarriers) equally spaced across an entire upstream RFspectrum (e.g. channel bandwidth of the symbol) in a pre-determined timeinterval (e.g. a symbol time). A CNU may transmit a pre-determinedwideband sequence (e.g. pilot sequence or probing sequence) in theprobing symbol by employing all pilot subcarriers in the probing symbol.When the CLT receives the probing symbol, the CLT may estimate upstreamchannel conditions between the CNU and the CLT at each of the pilotsubcarriers by comparing the received signal to the pre-determinedwideband sequence. In order to differentiate upstream transmissionsbetween different CNUs, the CLT may assign a separate probing symbol foreach CNU. However, channel bandwidth for upstream probing may increaseas the number of connected CNUs increases in a network, and thus mayresult in lower bandwidth efficiency. It should be noted that in thepresent disclosure, the terms upstream training and probing areequivalent and may be used interchangeably. In addition, the terms FCUand CLT are equivalent and may be used interchangeably

Disclosed herein is an upstream pilot scheme that may be performed by aP2MP OFDM communication system (e.g. a CLT or a CMTS) in a hybrid accessnetwork (e.g. an EPoC network or a DOCSIS network). A P2MP OFDMcommunication system may designate an upstream OFDM symbol as a probingsymbol for measuring upstream channels between a plurality of downstreamnetwork units and the P2MP OFDM communication system. In an embodiment,a CLT or a CMTS may assign a plurality of upstream training blocks froma probing symbol to a plurality of CNUs or CMs, respectively, where eachupstream training block may comprise a different subset of the pilotsubcarriers that are non-consecutive and span across the upstreamfrequency spectrum. A CNU or a CM may transmit a wideband pilot sequenceat the pilot subcarriers of an assigned upstream training block. The CNUor the CM may insert frequency nulls (e.g. values of zeroes) at theun-assigned pilot subcarriers (e.g. excluded subcarriers) so that theCNU or the CM may not interfere with other CNUs or CMs transmitting witha different set of pilot subcarriers in the same probing symbol. Assuch, the plurality of CNUs or CMs may transmit a different portion ofthe wideband pilot sequence at a different set of pilot subcarrierssimultaneously in the duration of the probing symbol. In an embodiment,the upstream training block may be specified in terms of a startingpilot subcarrier and a fixed number of subcarriers to skip betweensuccessive assigned pilot subcarriers. The disclosed upstream pilotscheme may utilize upstream bandwidth efficiently by allowing multipleCNUs or CMs to transmit simultaneously in a same probing symbol and mayprovide comparable upstream SNR performance as an upstream pilot schemethat designates one probing symbol per CNU or CM. In addition, thedisclosed upstream pilot scheme may allow a CMTS or CLT to probe a powerstarved (e.g. long distance and/or high attenuation channel) CNU or CMsuccessfully by employing only a subset of the subcarriers of the OFDMsymbol, where the CNU or CM may not have enough power to send a probingsequence with adequate power across all the subcarriers of the OFDMsymbol.

FIG. 1 is a schematic diagram of an embodiment of a unifiedoptical-coaxial network 100 comprising an optical portion 150 and acoaxial (electrical) portion 152. The network 100 may include an OLT110, at least one CNU 130 coupled to a plurality of subscriber devices140, and an CLT 120 positioned between the OLT 110 and the CNU 130,e.g., between the optical portion 150 and the coaxial portion 152. TheOLT 110 may be coupled via an ODN 115 to the CLTs 120, and optionally toone or more ONUs 170, or one or more HFC nodes 160 in the opticalportion 150. The ODN 115 may comprise fiber optics and an opticalsplitter 117 and/or a cascade of 1×M passive optical splitters thatcouple OLT 110 to the CLT 120 and any ONUs 170. The value of M in EPoC,e.g., the number of CLTs, may for example be 4, 8, 16, or other valuesand may be selected by the operator depending on factors such as opticalpower budget. The CLT 120 may be coupled to the CNUs 130 via anelectrical distribution network (EDN) 135, which may comprise a cablesplitter 137, a cascade of taps/splitters, and/or one or moreamplifiers. Each OLT 110 port may serve 32, 64, 128 or 256 CNUs 130. Itshould be noted that the upstream transmissions from CNUs 130 may reachthe CLT 120 and not the other CNUs 130 due to a directional property ofthe tap. The distances between the OLT 110 and the ONUs 170 and/or CLTs120 may range from about 10 to about 20 kilometers (km), and thedistances between the CLT 120 and CNUs 130 may range from about 100 toabout 500 meters (m). The network 100 may comprise any number of HFCs160, CLTs 120 and corresponding CNUs 130. The components of network 100may be arranged as shown in FIG. 1 or any other suitable arrangement.

The optical portion 150 of the network 100 may be similar to a PON inthat the optical portion 150 may be a communications network that doesnot require active components to distribute data between the OLT 110 andthe CLT 120. Instead, the optical portion 150 may use the passiveoptical components in the ODN 115 to distribute data between the OLT 110and the CLT 120. Examples of suitable protocols that may be implementedin the optical portion 150 may include asynchronous transfer mode PON(APON) or broadband PON (BPON) defined by the ITU TelecommunicationStandardization Sector (ITU-T) document G.983, Gigabit PON (GPON)defined by ITU-T document G.984, the EPON defined by IEEE documents802.3ah and 802.3av, all of which are incorporated by reference as ifreproduced in their entirety, the wavelength division multiplexing (WDM)PON (WDM-PON), and the Next Generation EPON (NGEPON) in development byIEEE.

The OLT 110 may be any device configured to communicate with the CNUs130 via the CLT 120. The OLT 110 may act as an intermediary between theCLTs 120 and/or CNUs 130 and another backbone network (e.g. theInternet). The OLT 110 may forward data received from a backbone networkto the CLTs 120 and/or CNUs 130 and forward data received from the CLTs120 or CNUs 130 onto the backbone network. Although the specificconfiguration of the OLT 110 may vary depending on the type of opticalprotocol implemented in the optical portion 150, in an embodiment, OLT110 may comprise an optical transmitter and an optical receiver. Whenthe backbone network employs a network protocol that is different fromthe protocol used in the optical portion 150, OLT 110 may comprise aconverter that may convert the backbone network protocol into theprotocol of the optical portion 150. The OLT converter may also convertthe optical portion 150 protocol into the backbone network protocol.

The ODN 115 may be a data distribution system that may comprise opticalfiber cables, couplers, splitters, distributors, and/or other equipment.In an embodiment, the optical fiber cables, couplers, splitters,distributors, and/or other equipment may be passive optical components.Specifically, the optical fiber cables, couplers, splitters,distributors, and/or other equipment may be components that do notrequire any power to distribute data signals between the OLT 110 and theCLT 120. It should be noted that the optical fiber cables may bereplaced by any optical transmission media in some embodiments. In someembodiments, the ODN 115 may comprise one or more optical amplifiers.The ODN 115 may extend from the OLT 110 to the CLT 120 and any optionalONUs 170 in a branching configuration as shown in FIG. 1, but may bealternatively configured as determined by a person of ordinary skill inthe art.

The CLT 120 may be any device or component configured to forwarddownstream data from the OLT 110 to the corresponding CNUs 130 andforward upstream data from the CNUs 130 to the OLT 110. The CLT 120 mayconvert the downstream and upstream data appropriately to transfer thedata between the optical portion 150 and the coaxial portion 152. Thedata transferred over the ODN 115 may be transmitted and/or received inthe form of optical signals, and the data transferred over the EDN 135may be transmitted and/or received in the form of electrical signalsthat may have the same or different logical structure as compared withthe optical signals. As such, the CLT 120 may encapsulate or frame thedata in the optical portion 150 and the coaxial portion 152 differently.In an embodiment, the CLT 120 may include a Media Access Control (MAC)layer and physical (PHY) layers, corresponding to the type of signalscarried over the respective media. The MAC layer may provide addressingand channel access control services to the PHY layers. As such, the PHYmay comprise an optical PHY and a coaxial PHY. In many embodiments, theCLT 120 may be transparent to the CNU 130 and OLT 110 in that the framessent from the OLT 110 to the CNU 130 may be directly addressed to theCNU 130 (e.g. in the destination address), and vice-versa. As such, theCLT 120 may intermediate between network portions, namely an opticalportion 150 and a coaxial portion 152 in the example of FIG. 1.

The ONUs 170 may be any devices that are configured to communicate withthe OLT 110 and may terminate the optical portion 150 of the network.The ONUs 170 may present customer service interfaces to end users. Insome embodiments, an ONU 170 may merge with a CLT 120 to form a FCU.

The electrical portion 152 of the network 100 may be similar to anyknown electrical communication system. The electrical portion 152 maynot require any active components to distribute data between the CLT 120and the CNU 130. Instead, the electrical portion 152 may use the passiveelectrical components in the electrical portion 152 to distribute databetween the CLT 120 and the CNUs 130. Alternatively, the electricalportion 152 may use some active components, such as amplifiers. Examplesof suitable protocols that may be implemented in the electrical portion152 include MoCA, G.hn, HPNA, and Home Plug A/V.

The EDN 135 may be a data distribution system that may compriseelectrical cables (e.g. coaxial cables, twisted wires, etc.), couplers,splitters, distributors, and/or other equipment. In an embodiment, theelectrical cables, couplers, splitters, distributors, and/or otherequipment may be passive electrical components. Specifically, theelectrical cables, couplers, splitters, distributors, and/or otherequipment may be components that do not require any power to distributedata signals between the CLT 120 and the CNU 130. It should be notedthat the electrical cables may be replaced by any electricaltransmission media in some embodiments. In some embodiments, the EDN 135may comprise one or more electrical amplifiers. The EDN 135 may extendfrom the CLT 120 to the CNU 130 in a branching configuration as shown inFIG. 1, but may be alternatively configured as determined by a person ofordinary skill in the art.

In an embodiment, the CNUs 130 may be any devices that are configured tocommunicate with the OLT 110, the CLT 120, and any subscriber devices140. The CNUs 130 may act as intermediaries between the CLT 120 and thesubscriber devices 140. For instance, the CNUs 130 may forward datareceived from the CLT 120 to the subscriber devices 140, and may forwarddata received from the subscriber devices 140 toward the OLT 110.Although the specific configuration of the CNUs 130 may vary dependingon the type of network 100, in an embodiment, the CNUs 130 may comprisean electrical transmitter configured to send electrical signals to theCLT 120 and an electrical receiver configured to receive electricalsignals from the CLT 120. Additionally, the CNUs 130 may comprise aconverter that may convert CLT 120 electrical signals into electricalsignals for the subscriber devices 140, such as signals in IEEE 802.11wireless local area network (WiFi) protocol. The CNUs 130 may furthercomprise a second transmitter and/or receiver that may send and/orreceive the converted electrical signals to the subscriber devices 140.In some embodiments, CNUs 130 and coaxial network terminals (CNTs) aresimilar, and thus the terms are used interchangeably herein. The CNUs130 may be typically located at distributed locations, such as thecustomer premises, but may be located at other locations as well.

The subscriber devices 140 may be any devices configured to interfacewith a user or a user device. For example, the subscribed devices 140may include desktop computers, laptop computers, tablets, mobiletelephones, residential gateways, televisions, set-top boxes, andsimilar devices.

FIG. 2 is a schematic diagram of an embodiment of a DOCSIS network 200,which may be structurally similar to the network 100. The DOCSIS network200 may be a DOCSIS 3.1 network as specified in DOCSIS 3.1 document,which is incorporated herein by reference as if reproduced in itsentirety. The network 200 may comprise a CMTS 210, at least one HFC node230, any number of CMs 250 and/or set-top box (STB) 252 arranged asshown in FIG. 2. Specifically, the HFC node 230 may be coupled to theCMTS 210 via an optical fiber 214, and the CMs 250 and/or STB 252 may becoupled to the HFC node 230 via electrical cables, one or moreamplifiers (e.g., amplifiers 236 and 238), and at least one splitter240. In implementation, the CMTS 210 may be substantially similar to theOLT 110, the HFC node 230 may be substantially similar to a CLT 130, anda CM 250 or a STB 252 may be substantially similar to a CNU 150. Itshould be noted that that the HFC node 230 may be remotely coupled tothe CMTS 210 or reside in the CMTS 210. In some embodiments, the CMTS210 may be equipped with part or all of the functionalities of the HFCnode 230.

It should be noted that present disclosure may describe an upstreampilot scheme in the context of an EPoC network (e.g. network 100) or aDOCSIS network (e.g. network 200). However, a person of ordinary skillin the art will recognize that the upstream pilot scheme describedherein may be applied to any network comprising a coaxial segment thatemploys P2MP OFDM transmission.

FIG. 3 is a schematic diagram of an embodiment of an NE 300, which mayact as a CLT (e.g. CLT 120) or a CMTS (e.g. CMTS 210) by implementingany of the schemes described herein. In some embodiments NE 300 may alsoact as other node(s) in the network, such as a media converter unit thatmay be coupled to an optical access network and an electrical wireless(e.g. WiFi) or wired network (e.g. coaxial, any Digital Subscriber Line(xDSL), powerline, etc) that employs OFDM transmission. One skilled inthe art will recognize that the term NE encompasses a broad range ofdevices of which NE 300 is merely an example. NE 300 is included forpurposes of clarity of discussion, but is in no way meant to limit theapplication of the present disclosure to a particular NE embodiment orclass of NE embodiments. At least some of the features/methods describedin the disclosure may be implemented in a network apparatus or componentsuch as an NE 300. For instance, the features/methods in the disclosuremay be implemented using hardware, firmware, and/or software installedto run on hardware. As shown in FIG. 3, the NE 300 may comprisetransceivers (Tx/Rx) 310, which may be transmitters, receivers, orcombinations thereof. A Tx/Rx 310 may be coupled to plurality ofdownstream ports 320 for transmitting and/or receiving frames from othernodes and a Tx/Rx 310 may be coupled to plurality of upstream ports 350for transmitting and/or receiving frames from other nodes, respectively.A processor 330 may be coupled to the Tx/Rx 310 to process the framesand/or determine which nodes to send the frames to. The processor 330may comprise one or more multi-core processors and/or memory devices332, which may function as data stores, buffers, etc. Processor 330 maybe implemented as a general processor or may be part of one or moreapplication specific integrated circuits (ASICs) and/or digital signalprocessors (DSPs). Processor 330 may comprise an OFDM upstream trainingmodule 331, which may implement an upstream training method, such asmethod 700 or 800 at a CLT, a CMTS, or any other network nodes thatperform upstream training for OFDM transmission, such as a CNU or CM. Inan alternative embodiment, the OFDM upstream training module 331 may beimplemented as instructions stored in the memory devices 332, which maybe executed by processor 330. The memory device 332 may comprise a cachefor temporarily storing content, e.g., a Random Access Memory (RAM).Additionally, the memory device 332 may comprise a long-term storage forstoring content relatively longer, e.g., a Read Only Memory (ROM). Forinstance, the cache and the long-term storage may include dynamic randomaccess memories (DRAMs), solid-state drives (SSDs), hard disks, orcombinations thereof.

It is understood that by programming and/or loading executableinstructions onto the NE 300, at least one of the processor 330 and/ormemory device 332 are changed, transforming the NE 300 in part into aparticular machine or apparatus, e.g., a multi-core forwardingarchitecture, having the novel functionality taught by the presentdisclosure. It is fundamental to the electrical engineering and softwareengineering arts that functionality that can be implemented by loadingexecutable software into a computer can be converted to a hardwareimplementation by well-known design rules. Decisions betweenimplementing a concept in software versus hardware typically hinge onconsiderations of stability of the design and numbers of units to beproduced rather than any issues involved in translating from thesoftware domain to the hardware domain. Generally, a design that isstill subject to frequent change may be preferred to be implemented insoftware, because re-spinning a hardware implementation is moreexpensive than re-spinning a software design. Generally, a design thatis stable that will be produced in large volume may be preferred to beimplemented in hardware, for example in an ASIC, because for largeproduction runs the hardware implementation may be less expensive thanthe software implementation. Often a design may be developed and testedin a software form and later transformed, by well-known design rules, toan equivalent hardware implementation in an ASIC that hardwires theinstructions of the software. In the same manner as a machine controlledby a new ASIC is a particular machine or apparatus, likewise a computerthat has been programmed and/or loaded with executable instructions maybe viewed as a particular machine or apparatus.

In an embodiment, OFDM transmission may be employed in a coaxial networkor a hybrid access network (e.g. network 100 and/or 200) that comprisesa coaxial segment. In OFDM transmission, digital data may be encodedonto multiple orthogonal subcarrier signals and transmitted in terms ofOFDM symbols. An OFDM symbol may be defined as a group of frequencysubcarriers equally spaced across an RF spectrum for communications in apre-determined time interval (e.g. a symbol time duration). An OFDMframe may be defined as a group of pre-determined number of OFDM symbolsthat spans in time and frequency. A central network access unit (e.g. aP2MP OFDM communication system, CLT 120, CMTS 210) may designate an OFDMframe as a probing frame for upstream channel measurements (e.g.probing). The OFDM symbols within a probing frame may be referred to asprobing symbols and the subcarriers within a probing symbol may bereferred to as pilot subcarriers or pilots.

The central access network unit may divide a probing symbol into aplurality of upstream training blocks. For example, each upstreamtraining block may comprise a different subset of the pilot subcarriers(e.g. assigned pilot subcarriers) scattered across an entire channelbandwidth of the probing symbol with skipped subcarriers (e.g.un-assigned pilot subcarriers) between the successive assigned pilotsubcarriers. As such, the pilot subcarriers in an upstream trainingblock may be non-consecutive (e.g. skipping some pilot subcarriers) infrequency, but may span across the entire upstream spectrum. The centralaccess network unit may assign one or more of the upstream trainingblocks in a single probing symbol to one or more connected downstreamnetwork units (e.g. CNUs 130, CMs 250).

Each downstream network unit may transmit the pre-determined sequenceaccording to the assigned upstream training block to enable upstreamchannel training, where the pre-determined sequence may be referred toas a pilot sequence, a probing sequence, or a wideband pilot sequence.For example, each downstream network unit may modulate a pilot sequenceaccording to a Binary Phase Shift Keying (BPSK) modulation scheme into aseries of BPSK symbols, map one BPSK symbol onto one pilot subcarrier inthe probing symbol, and set the un-assigned pilot subcarriers to zeroes(e.g. frequency nulls). As such, each downstream network unit maytransmit a different portion of the pilot sequence at a different subsetof the pilot subcarriers (e.g. assigned pilot subcarriers) and transmitfrequency nulls at the un-assigned pilot subcarriers, where theun-assigned pilot subcarriers may be assigned to other downstreamnetwork units. Thus, the simultaneous transmissions of the probingsymbol from one downstream network unit may not interfere with anotherdownstream network unit.

When the central access network unit receives the probing symbol, thecentral access network unit may compute an upstream channel response foreach of the downstream network units that transmitted one or more of theassigned upstream training blocks in the probing symbol. For example,the central access network unit may compute an upstream channel estimatefor a downstream network unit by comparing the received signal with thepre-determined pilot sequence at the assigned pilot subcarriers of theone or more upstream training blocks corresponding to the downstreamnetwork unit and interpolating the computed channel estimates to obtainchannel estimates at the frequency subcarriers that are excluded fromthe one or more assigned upstream training blocks.

In an embodiment of upstream training, a central access network unit maydetermine upstream pre-equalizer taps (e.g. coefficients) according toan upstream channel response estimated for a downstream network unit andmay transmit the pre-equalizer coefficients to the downstream networkunit. The downstream network unit may apply an upstream pre-equalizerwith the received coefficients prior to transmitting a signal to thecentral access network unit. As such, the central access network unitmay receive a signal with a flat response (e.g. with channel distortionpre-compensated) from the downstream network unit, and thus may simplifyupstream channel equalization.

In another embodiment of upstream training, a central access networkunit may measure SNR for each subcarrier (e.g. per tone SNR) anddetermine an appropriate bit loading (e.g. number of data bits) for eachsubcarrier according to the measured SNR. For example, the centralaccess network unit may assign a higher order modulation scheme (e.g. 64Quadrature Amplitude Modulation (QAM) with six bits per tone, 256 QAMwith eight bits per tone) for a high SNR subcarrier and a lower ordermodulation scheme (e.g. BPSK with one bit per tone) for a low SNRsubcarrier. In addition, the central access network unit may dynamicallyadjust the bit loading for each subcarrier to adapt to changes inupstream channel conditions (e.g. varying SNRs).

FIG. 4 is a schematic diagram of an embodiment of a probing symbol 400comprising one upstream training block 410 that spans the entire probingsymbol 400. The probing symbol 400 may comprise a plurality of pilotsubcarriers 411. For example, probing symbol 400 may comprise 4096 pilotsubcarriers for a 4K Fast Fourier Transform (FFT), 2048 pilotsubcarriers for a 2K FFT, etc. The upstream training block 410 may beassigned with all the 4096 pilot subcarriers 411 (e.g. activesubcarriers) without skipping subcarriers. As such, the upstreamtraining block 410 may be employed to transmit a wideband pilot sequenceat the pilot subcarriers 411 (e.g. from subcarrier zero to 4095 for 4KFFT) in the probing symbol 400.

FIG. 5 is a schematic diagram of another embodiment of a probing symbol500 comprising an upstream training block 510. The probing symbol 500may comprise a plurality of pilot subcarriers 511 and 521. For example,probing symbol 500 may comprise 4096 pilot subcarriers for a 4K FFT,2048 pilot subcarriers for a 2K FFT, etc. The upstream training block510 may be assigned with alternating pilot subcarriers 511 and notassigned with pilot subcarriers 521 by skipping one subcarrier 521between successive assigned pilot subcarriers 511. The skippedsubcarriers 521 may be skipped for various reasons, for example, anothersystem may be transmitting on the excluded subcarriers 521. As such, theupstream training block 510 may be employed to transmit a portion of awideband pilot sequence at alternating pilot subcarriers 511 (e.g.assigned pilot subcarriers) in the probing symbol 500.

FIG. 6 is a schematic diagram of another embodiment of a probing symbol600 comprising two upstream training blocks 610 and 620. The probingsymbol 600 may comprise a plurality of pilot subcarriers 611 and 621.For example, probing symbol 600 may comprise 4096 pilot subcarriers fora 4K FFT, 2048 pilot subcarriers for a 2K FFT, etc. The upstreamtraining block 610 may start at the lowest frequency subcarrier (e.g.subcarrier zero) and comprise every second pilot subcarriers 611 in theprobing symbol 600. The upstream training block 620 may start at thenext lowest frequency subcarrier (e.g. subcarrier one) and compriseevery second pilot subcarriers 621 in the probing symbol 600. Thus, eachupstream training block 610 or 620 may be employed to transmit adifferent portion of a wideband pilot sequence at the pilot subcarriers611 or 621, respectively. As such, the upstream training blocks 610 and620 may interleave in frequencies, but may not overlap in frequencies.It should be noted that a central access network unit (e.g. CLT 120,CMTS 210) may assign the upstream training blocks 610 and 620 to twodifferent downstream network units (e.g. CNUs 130, CMs 250), forexample, the central access network unit may assign the upstreamtraining block 610 to a downstream network unit A and the upstreamtraining block 620 to a downstream network unit B. Thus, a centralaccess network unit may assign M upstream training blocks to Mdownstream network units, where each upstream training block maycomprise a different set of pilot subcarriers and the successive pilotsubcarriers in a upstream training block may be separated by M-1subcarriers.

In an embodiment of an EPoC network, such as network 100, a CLT (e.g.CLT 120) may allocate a specific probing symbol to a CNU within aprobing frame and instruct the CNU (e.g. CNU 130) to transmit a probingsequence in the symbol. The CLT may assign the CNU all the pilots or asubset of (e.g. scattered) pilots of the assigned probing symbol. TheCNU may transmit pilots spanning all active subcarriers during upstreamwideband probing. The CNU may transmit one pilot per subcarrier. Eachpilot may be a pre-defined BPSK symbol. The OFDM symbol which is usedfor probing may be defined as a probing symbol. The CLT may employ thereceived probing symbol upstream channel estimation and/or upstream SNRmeasurements. For example, the CLT may compute coefficients of anupstream pre-equalizer for each CNU and send the coefficients back tothe corresponding CNU. In addition, the CLT may measure SNR persubcarrier and compute an upstream bit loading table for each CNU. Itshould be noted that a CNU may not transmit a probing sequence in anexcluded subcarrier. The excluded subcarriers may be the subcarriers inwhich no CNU may be allowed to transmit because the excluded subcarriersmay be at frequencies employed by other systems (e.g. includingguard-band subcarriers). The probing pattern may continue un-interruptedin presence of excluded subcarriers and/or guard bands. However, the CNUmay not transmit any pilots in the excluded subcarrier and/or guardbands.

In an embodiment of a DOCSIS network, such as network 200, upstreamwideband probing may be employed during admission and steady state forpre-equalization configuration and periodic transmission power andtime-shifting ranging. In a DOCSIS network, a CMTS (e.g. CMTS 210) maydesignate an OFDM frame for upstream probing, where the probing framemay comprise K contiguous probing symbols (e.g. OFDM symbols), where Kis the number of symbols in a minislot (e.g. a group of subcarriers inthe K number of symbols). The probing frame may be aligned with theminislot boundaries in a time domain. A probing symbol may comprisepilots that are BPSK subcarriers, generated from a Pseudo Random BinarySequence (PRBS) generation scheme, which may be discussed more fullybelow. A CMTS may allocate a specific probing symbol within a probingframe to a CM (e.g. CM 250) and instruct the CM to transmit a probingsequence in the probing symbol. The CMTS may define a probing patterncomprising pilots from all the subcarriers of the assigned probingsymbol or a set of pilots from scattered subcarriers of the assignedprobing symbol. A CM may generate a wideband pilot sequence according tothe PRBS generation scheme to generate 2048 or 4096 subcarriers for a 2KFFT or 4K FFT, respectively. The CM may employ the same BPSK modulationfor a specific subcarrier in all probing symbols. The CM may nottransmit a probing sequence in an excluded subcarrier. The CM maytransmit zero valued subcarriers in the excluded subcarriers. Excludedsubcarriers may be subcarriers in which no CM may be allowed to transmitbecause the excluded subcarriers may be at frequencies employed by othersystem (e.g. including guard-band subcarriers).

In an embodiment, a wideband pilot sequence may be generated by apre-determined PRBS generation scheme. For example, the polynomialdefinition for the PRBS scheme may be as shown below:

X ¹² +X ⁹ +X ⁸ +X ⁵+1

where a seed of 3071 and a period of 2¹²-1 bits may be employed. Theperiod of 2¹²-1 bits may be sufficient to create one probing symbolwithout repetitions. The wideband pilot sequence may be mapped to BPSKpilots. For example, a value of zero may be mapped to a BPSK pilot ofone and a value of one may be mapped to a BPSK pilot of minus one. Assuch, the probing symbol pilots are BPSK symbols. A probing pilot i maybe associated with the i-th subcarrier of the symbol, where

i=0,1, . . . , 4095 for a 4K FFT

and

i=0,1, . . . , 2047 for a 2K FFT

It should be noted that the subcarriers may be numbered in ascendingorder starting from zero.

In an embodiment, a central access network unit may assign an upstreamtraining block by specifying a symbol number for upstream probing, astarting pilot subcarrier number (e.g. ranges from zero to seven) and anumber of subcarriers to skip between successive pilot subcarriers inthe symbol. The central access network unit may send the upstreamtraining block assignment in a message (e.g. an upstream bandwidthallocation map (MAP) message). For example, the symbol number may bespecified in terms of a number of symbols offset a start of a probingframe and the probing frame may be specified in terms of a number ofOFDM frames offset from the beginning of a frame that corresponds to anallocation start time specified in the message.

In an embodiment of an EPoC network, such as network 100, a CLT (e.g.CLT 120) may specify a probing symbol within a probing frame through aSymbol In Frame parameter. The CLT may allocate subcarriers within theprobing symbol by sending two parameters to a CNU (e.g. CNU 130), astart subcarrier parameter and a subcarrier skipping parameter. Thestart subcarrier parameter may refer to a starting subcarrier number andmay comprise values ranging from about zero to about seven. Thesubcarrier skipping parameter may refer to the number of subcarriers tobe skipped between successive pilots and may comprise values rangingfrom about zero to about seven. A value of zero for the skippingsubcarrier (e.g. subcarrier skipping=0) may refer to no skipping ofsubcarriers (e.g. all subcarriers may be used for probing). For example,the upstream training block 410 in the probing symbol 400 may bespecified with a starting subcarrier parameter value of zero and asubcarrier skipping parameter of zero. Similarly, the upstream trainingblock 510 in the probing symbol 500 may be specified with a startingsubcarrier parameter value of zero and a subcarrier skipping parameterof one. A CLT may specify the upstream training block 610 with astarting subcarrier parameter value of zero and a skipping parametervalue of one when assigning the upstream training block to a downstreamnetwork unit A. Similarly, a CLT may specify the upstream training block620 with a starting subcarrier parameter value of one and a skippingparameter value of one when assigning the upstream training block to adownstream network unit B.

In an embodiment of a DOCSIS network, such as network 200, a CMTS (e.g.CMTS 210) may specify a probing symbol within a probing frame through aparameter Symbol In Frame and may specify additional parameters, such asa start subcarrier parameter and a subcarrier skipping parameter. Thestart subcarrier parameter may refer to a starting subcarrier number andthe start subcarrier parameter value may range from about zero to aboutseven. The skipping subcarrier parameter may refer to the number ofsubcarriers to be skipped between successive pilot and the skippingsubcarrier parameter value may range from about zero to about seven. Askipping subcarrier parameter value of zero (e.g. skipping subcarrier=0)may refer to no skipping of subcarriers, for example, all subcarriers ina single symbol may belong to a single transmitter. In such anembodiment, a CM may employ the start subcarrier and subcarrier skippingparameters to determine which subcarriers may be employed for probingtransmission.

FIG. 7 is a flowchart of an embodiment of an upstream training method700. Method 700 may be implemented by a central access network unit(e.g. CLT 120, CMTS 210, and/or NE 300) during upstream training. Method700 may begin with allocating an OFDM symbol for upstream training atstep 710. At step 720, method 700 may divide the OFDM symbol into aplurality of upstream training blocks, where each upstream trainingblock may be specified in terms of a starting subcarrier number (e.g. afirst assigned pilot subcarrier) and a number of subcarriers to skipbetween the successive pilot subcarriers. For example, each upstreamtraining block may comprise a different starting subcarrier number, butmay comprise the same number of skipping subcarriers. As such, theupstream training blocks may comprise a different set of pilotsubcarriers that are non-consecutive pilot subcarriers and span acrossthe upstream frequency spectrum.

At step 730, method 700 may assign the upstream training blocks to oneor more downstream network units. At step 740, method 700 may generate amessage indicating the assignments of the upstream training blocks. Forexample, each assignment may comprise an identifier that identifies adownstream network unit for the assignment, a probing frame number (e.g.OFDM frame offset from an allocation start time), a symbol number in aprobing frame (e.g. OFDM symbol offset from a start of an OFDM frame), astarting subcarrier number (e.g. subcarrier offset from a lowestfrequency of an OFDM symbol), and a number of skipping subcarriersbetween successive pilot subcarriers. It should be noted that in someembodiments, method 700 may generate more than one message to indicatethe assignments of the upstream training blocks depending on theemployed message protocol.

At step 750, method 700 may send the message to the downstream networkunits. After sending the message to the one or more downstream networkunits, method 700 may wait for the assigned probing symbol to bereceived from the downstream network units at step 760. Upon receivingthe probing symbol, method 700 may perform upstream channel estimationand SNR measurements at step 770. For example, method 700 may compute anupstream channel estimate for each downstream network unit at the pilotsubcarriers of an upstream training block assigned to the downstreamnetwork unit by comparing the received signal value to a pre-determinedsequence (e.g. specified by a standard body or a network configuration).After computing the channel estimates at the pilot subcarriers of theupstream training block assigned to the downstream network unit, method700 may interpolate the computed channel estimates to obtain channelestimates for the skipped subcarriers. It should be noted that method700 may be applied dynamically or periodically for upstream channelmeasurement such that upstream transmissions may be adapted to channelvariations.

In an embodiment of an EPoC network, such as network 100, a CLT (e.g.CLT 120) may schedule a single CNU (e.g. CNU 130) in a probing symbolwithout skipping subcarriers (e.g. upstream training block 410 inprobing symbol 400). In such embodiment, the CLT may allocate a specificprobing symbol to a single CNU, and may set a subcarrier skippingparameter value to zero and a starting subcarrier parameter value to anumber of the first subcarrier in the probing symbol.

In an alternative embodiment of an EPoC network, such as network 100, aCLT (e.g. CLT 120) may schedule a single CNU (e.g. CNU 130) in a probingsymbol with skipping subcarriers to create nulls (e.g. upstream trainingblock 510 in probing symbol 500). In such an embodiment, the CLT mayallocate a specific probing symbol to a single CNU, and may set asubcarrier skipping parameter value to a non-zero positive integer valueand a start subcarrier parameter value to a number of the firstsubcarrier in the probing symbol.

In yet another alternative embodiment of an EPoC network, such asnetwork 100, a CLT (e.g. CLT 120) may schedule multiple CNUs (e.g. CNUs130) in a probing symbol (e.g. probing symbol 600). In such anembodiment, the CLT may allocate the same probing symbol at any giventime to more than one CNU. The CLT may assign a different startsubcarrier to each CNU and the same subcarrier skipping value to everyCNU within the probing symbol. It should be noted that in such anembodiment, the CLT may or may not assign skipping subcarriers to createnulls, for example, the CLT may create nulls by specifying a subcarrierskipping value equal to or greater than the number of CNUs in thepattern.

FIG. 8 is a flowchart of another embodiment of an upstream trainingmethod 800. Method 800 may be implemented by a downstream network unit(e.g. CNU 130, CM 250, and/or NE 300) during upstream training. Method800 may begin with receiving an upstream training block assignment for aspecific probing symbol in step 810. For example, the upstream trainingblock assignment may indicate a symbol number (e.g. offset from a startof an OFDM frame) for the probing symbol, a starting subcarrier number(e.g. a first assigned pilot subcarrier) and a number of skippingsubcarriers between successive assigned pilot subcarriers in the probingsymbol. At step 820, method 800 may generate a pre-determined sequenceaccording to a pre-determined generation scheme (e.g. a PRBS scheme). Atstep 830, method 800 may generate the probing symbol in a frequencydomain by modulating the generated sequence onto the assignedsubcarriers of the probing symbol. At step 840, method 800 may set theskipped subcarriers to values of zeroes. At step 850, method 800 mayperform an inverse Fast Fourier Transform (IFFT) to transform theprobing symbol to a time domain. The step 860, method 800 may transmitthe probing symbol at a time specified by the assignment.

FIG. 9 is a schematic diagram of an embodiment of an upstream trainingmessage encoding 900. The upstream training message structure 900 may betransmitted by a central access network unit (e.g. CLT 120, CMTS 210) toone or more downstream network units (e.g. CNUs 130, CMs 250) in ahybrid access network (e.g. network 100, 200) to indicate usage ofsymbols in a probing frame. For example, the upstream training messagestructure 900 may be embedded in a MAP message. The message structure900 may comprise a plurality of successive probing information elements(P-IEs) 910 that describe the specific usage of symbols within a probingframe (e.g. one P-IE 910 per probing symbol). Each P-IE 910 may be aboutthirty two bits in length and the bits within the P-IE 910 may benumbered from bit position zero to bit position thirty one. Each P-IE910 may comprise a service flow identifier (SID) field 911, a reserved(R) field 912, a power (PW) field 913, an equalizer (EQ) field 914, astagger (St) field 915, a probing frame (PrFr) field 916, a Symbol InFrame field 917, a start subcarrier (Start Subc) field 918, and asubcarrier skip (Subc Skip) field 919. It should be noted that thecentral access network unit may indicate the successive P-IE 910 inmessage structure 900 in a time-order (e.g. earliest symbol first) andsubcarrier order (e.g. lowest subcarrier first). In addition, a probingframe may comprise a combination of allocation probing symbols andunallocated probing symbols.

The SID field 911 may be about fourteen bits in length and may extendfrom bit position zero to bit position thirteen. The SID field 911 maycomprise data indicating a ranging SID for a downstream network unitassigned to use the P-IE 910. The R field 912 may be about two bits inlength and may extend from bit position fourteen to bit positionfifteen. The R field 912 may be reserved for future extension.

The PW field 913 may be about one bit in length and may be positioned atbit position fifteen. The PW field 913 may indicate whether powercontrol may be employed for probing. For example, the PW field 913 maybe set to a value of zero to instruct a downstream network unitidentified by the SID specified in the SID field 911 to transmit withnormal power settings and set to a value of one to instruct thedownstream network unit to transmit with modified power settingcommunicated in a previous ranging response (RNG-RSP) message.

The EQ field 914 may be about one bit in length and may be positioned atbit position sixteen. The EQ field 914 may indicate whether a transmitequalizer may be employed for probing. For example, the EQ field 914 maybe set to a value of zero to instruct a downstream network unitidentified by the SID specified in the SID field 911 to enable thetransmit equalizer and set to a value of one to instruct the downstreamnetwork unit to disable the transmit equalizer.

The St field 915 may be about one bit in length and may be positioned atbit position seventeen. The St field 915 may indicate whether astaggered pattern may be employed for pilot subcarriers. For example,the St field 915 may be set to a value of one to instruct a downstreamnetwork unit identified by the SID specified in the SID field 911 torepeat a pattern in P-IE 910 in the next number of symbols equal inquantity to Subc Skip field 919 and by moving the pattern up by onesubcarrier in each symbol and wrapping the pattern back to thebeginning. Alternatively, the St field 915 may be set to a value of zeroto instruct the downstream network unit employ pilot subcarriers withouta staggered pattern.

The PrFr field 916 may be about two bits in length and may extend frombit position eighteen to bit position nineteen. The PrFr field 916 maycomprise data indicating a number of frames offset from a framebeginning at an allocation start time specified in a MAP message thatcarries the message structure 900 and may indicate the first frame forwhich the P-IE 910 is applicable. For example, the PrFr field 916 may beset to a value of zero to indicate a first probing frame of the MAP.

The Symbol In Frame field 917 may be about six bits in length and mayextend from bit position twenty to bit position twenty five. The SymbolIn Frame field 917 may comprise data indicating a number of symbolsoffset from the beginning of a probing frame specified in the PrFr field915. For example, the Symbol In Frame field 917 may comprise a valueranging from zero to thirty five and a value of zero may indicate afirst symbol of the probing frame.

The Start Subc field 918 may be about three bits in length and mayextend from bit position twenty six to bit position twenty eight. TheStart Subc field 918 may comprise data indicating a starting subcarrierto be employed by probing. For example, the Start Subc field 918 may beset to a value of zero to indicate a first subcarrier in a symbolspecified by the Symbol In Frame field 917.

The Subc Skip field 919 may be about three bits in length and may extendfrom bit position twenty nine to bit position thirty one. The Subc Skipfield 919 may comprise data indicating a number of subcarriers to beskipped between successive pilots in a probe. For example, the Subc Skipfield 919 may be set to a value of zero to indicate no skipping ofsubcarriers and that all non-excluded subcarriers may be employed forprobing. It should be noted that the Subc Skip field 919 may indicateadditional information when staggering is employed. For example, thevalue of the Subc Skip file 919 plus one may indicate a total number ofsymbols for which the staggered P-IE allocation may be applied in theprobing frame.

FIG. 10 illustrates a graph 1000 of an embodiment of upstream SNR lossas a function of number of probing downstream network units in a singleprobing symbol. The x-axis may represent a number of probing downstreamnetwork units per probing symbol and the y-axis may represent SNR lossin units of decibels (dBs) when compared to probing a single downstreamnetwork unit. In graph 1000, curves 1010, 1020, 1030, 1040, and 1050 mayrepresent upstream SNR loss versus number of downstream network unitsprobed in a single probing symbol for an Additive White Gaussian Noise(AWGN) channel of 35 dB, 30 dB, 25 dB, 20 dB, and 15 dB, respectively.As can be observed from the curves 1010, 1020, 1030, 1040, and 1050, theSNR loss from probing up to about four downstream network units in asingle symbol may be minimal and the SNR may be comparable to probingone downstream network unit per probing symbol. However, the SNR maygradually degrade as the number of downstream network units increasesand the rate of degradation may vary depending on channel conditions.For example, the SNR may degrade at a slower rate (e.g. slope of curve1050, about 0.1 dB SNR loss for ten probing downstream network units)for a low SNR channel as channel noise may be dominated by the AWGN.Conversely, the SNR may degrade at a faster rate (e.g. slope of curve1010, about 3.5 dB SNR loss for ten probing downstream network units)for a high SNR channel (e.g. AWGN of 35 dB) as channel noise may bedominated by inaccuracies of upstream channel estimates when multipledownstream network units are probed in a single probing symbol.

At least one embodiment is disclosed and variations, combinations,and/or modifications of the embodiment(s) and/or features of theembodiment(s) made by a person having ordinary skill in the art arewithin the scope of the disclosure. Alternative embodiments that resultfrom combining, integrating, and/or omitting features of theembodiment(s) are also within the scope of the disclosure. Wherenumerical ranges or limitations are expressly stated, such expressranges or limitations should be understood to include iterative rangesor limitations of like magnitude falling within the expressly statedranges or limitations (e.g. from about 1 to about 10 includes, 2, 3, 4,etc.; greater than 0.10 includes 0.11, 0.12, 0.13, etc.). For example,whenever a numerical range with a lower limit, R₁, and an upper limit,R_(u), is disclosed, any number falling within the range is specificallydisclosed. In particular, the following numbers within the range arespecifically disclosed: R=R₁+k*(R_(u)−R₁), wherein k is a variableranging from 1 percent to 100 percent with a 1 percent increment, i.e.,k is 1 percent, 2 percent, 3 percent, 4 percent, 7 percent, . . . , 70percent, 71 percent, 72 percent, . . . , 97 percent, 96 percent, 97percent, 98 percent, 99 percent, or 100 percent. Moreover, any numericalrange defined by two R numbers as defined in the above is alsospecifically disclosed. Unless otherwise stated, the term “about”means±10% of the subsequent number. Use of the term “optionally” withrespect to any element of a claim means that the element is required, oralternatively, the element is not required, both alternatives beingwithin the scope of the claim. Use of broader terms such as comprises,includes, and having should be understood to provide support fornarrower terms such as consisting of, consisting essentially of, andcomprised substantially of. Accordingly, the scope of protection is notlimited by the description set out above but is defined by the claimsthat follow, that scope including all equivalents of the subject matterof the claims. Each and every claim is incorporated as furtherdisclosure into the specification and the claims are embodiment(s) ofthe present disclosure. The discussion of a reference in the disclosureis not an admission that it is prior art, especially any reference thathas a publication date after the priority date of this application. Thedisclosure of all patents, patent applications, and publications citedin the disclosure are hereby incorporated by reference, to the extentthat they provide exemplary, procedural, or other details supplementaryto the disclosure.

While several embodiments have been provided in the present disclosure,it should be understood that the disclosed systems and methods might beembodied in many other specific forms without departing from the spiritor scope of the present disclosure. The present examples are to beconsidered as illustrative and not restrictive, and the intention is notto be limited to the details given herein. For example, the variouselements or components may be combined or integrated in another systemor certain features may be omitted, or not implemented.

In addition, techniques, systems, subsystems, and methods described andillustrated in the various embodiments as discrete or separate may becombined or integrated with other systems, modules, techniques, ormethods without departing from the scope of the present disclosure.Other items shown or discussed as coupled or directly coupled orcommunicating with each other may be indirectly coupled or communicatingthrough some interface, device, or intermediate component whetherelectrically, mechanically, or otherwise. Other examples of changes,substitutions, and alterations are ascertainable by one skilled in theart and could be made without departing from the spirit and scopedisclosed herein.

What is claimed is:
 1. A central access network unit comprising: aprocessor configured to: assign a plurality of upstream training blocksfrom an upstream orthogonal frequency division multiplexing (OFDM)symbol to a plurality of downstream network units, wherein the OFDMsymbol comprises a plurality of pilot subcarriers equally spaced acrossan upstream radio frequency (RF) spectrum in a pre-determined timeinterval, and wherein each upstream training block comprises a differentsubset of the pilot subcarriers that are non-consecutive and situatedacross the upstream RF spectrum; and generate one or more messagescomprising assignments of the upstream training blocks; and atransmitter coupled to the processor and configured to transmit themessages to the plurality of downstream network units via a network,wherein the messages instruct at least one of the plurality ofdownstream network units to transmit a modulated pre-determined sequenceat the pilot subcarriers corresponding to the upstream training blockassigned to the downstream network unit.
 2. The central access networkunit of claim 1, wherein each upstream training block assignmentcomprises: a first parameter indicating a starting pilot subcarrierposition for the subset of pilot subcarriers in the upstream trainingblock in the upstream OFDM symbol; and a second parameter indicating anumber of excluded subcarriers between successive pilot subcarriers inthe subset of pilot subcarriers.
 3. The central access network unit ofclaim 2, wherein each upstream training block comprises a differentstarting pilot subcarrier position and a same number of excludedsubcarriers.
 4. The central access network unit of claim 1 furthercomprising a receiver coupled to the processor and configured to receivethe upstream OFDM symbol comprising at least one of the plurality ofupstream training blocks from at least one of the plurality ofdownstream network units via the network, wherein the received OFDMsymbol comprises the modulated pre-determined sequence at the pilotsubcarriers of the upstream training block, and wherein the processor isfurther configured to compute an upstream channel response between atleast one of the plurality of downstream network units and the centralnetwork unit by processing the pilot subcarriers according to theupstream training block assigned to the downstream network unit.
 5. Thecentral access network unit of claim 4, wherein the processor is furtherconfigured to: determine coefficients for an upstream pre-equalizeraccording to the upstream channel response of the downstream networkunit; and send the coefficients to the downstream network unit.
 6. Thecentral access network unit of claim 1, wherein the processor is furtherconfigured to: measure upstream Signal-to-Noise (SNR) for at least oneof the plurality of downstream network units from the received upstreamOFDM symbol; and determine an upstream modulation scheme for the atleast one downstream network unit according to the measured upstreamSNR.
 7. The central access network unit of claim 1, wherein the networkis an Ethernet passive optical network over coax (EPoC) network, whereinthe central access network unit is a Coaxial Line Terminal (CLT), andwherein the downstream network units are Coaxial Network Units (CNUs).8. The central access network unit of claim 1, wherein the network is adata over cable service interface specification (DOCSIS) network,wherein the central access network unit is a Cable Modem TerminationSystem (CMTS), and wherein the downstream network units are Cable Modems(CMs).
 9. A method implemented by a Cable Modem Termination System(CMTS) comprising: allocating, by the CMTS, a specific probing symbolwithin a probing frame for wideband probing; defining a probing patternin the allocated probing symbol, wherein the probing pattern comprises aset of pilots from scattered subcarriers of the allocated probingsymbol; and instructing a Cable Modem (CM) to transmit a probingsequence in the allocated probing symbol.
 10. The method of claim 9further comprising instructing the CM to transmit zero valuedsubcarriers in exclusion subcarriers.
 11. The method of claim 9, whereininstructing the CM to transmit the probing sequence comprises sending anupstream bandwidth allocation map (MAP) message comprising: a Symbol InFrame parameter that specifies the probing symbol within the probingframe, wherein the Symbol In Frame parameter is a number of symbolsoffset from the beginning of the probing frame; a starting subcarrierparameter that indicates a starting subcarrier to be used by the probepattern; and a subcarrier skipping parameter that is a number ofsubcarriers to be skipped between successive pilots in the probepattern.
 12. The method of claim 11, wherein the probing symbolcomprises a 2048-points (2K) Fast Fourier Transform (FFT) or a4096-points (4K) FFT, wherein the starting subcarrier parameter rangesfrom zero to seven, and wherein the subcarrier skipping parameter rangesfrom zero to seven.
 13. The method of claim 9 further comprising:performing pre-equalization configuration; and performing periodictransmission power and time-shift ranging.
 14. The method of claim 9,wherein the probing frame comprises K contiguous probing symbols thatare Orthogonal Frequency Division Multiplexing (OFDM) symbols, wherein Kis a number of symbols in a minislot, and wherein the probing frame isaligned with the minislot boundaries in a time domain.
 15. A methodimplemented by a Coaxial Line Terminal (CLT) comprising: allocating, bythe CLT, a specific probing symbol to a Coaxial Network Unit (CNU)within a probing frame for upstream wideband probing; assigning the CNUa subset of scattered pilots of the probing symbol; receiving theallocated probing symbol from the CNU; and performing upstream channelestimation by using the received probing symbol.
 16. The method of claim15, wherein performing upstream channel estimation comprises: computingcoefficients of an upstream pre-equalizer for the CNU; and sending thecomputed coefficients to the CNU, and wherein the method furthercomprises: measuring upstream Signal-to-Noise Ratio (SNR) per subcarrierby using the received probing symbol; and computing an upstream bitloading table for the CNU according to the measured SNR.
 17. The methodof claim 15 further comprising: specifying the probing symbol within theprobing frame through a parameter Symbol In Frame; and instructing theCNU to transmit a probing sequence in the allocated probing symbol andnot to transmit the probing sequence in an excluded subcarrier by:sending a starting subcarrier parameter that is a starting subcarriernumber; and sending a subcarrier skipping parameter that is a number ofsubcarriers to be skipped between successive pilots.
 18. The method ofclaim 17, wherein the probing symbol comprises a 4096-points (4K) FastFourier Transform (FFT) with the subcarriers numbered in an ascendingorder starting from zero, wherein the starting subcarrier parameterranges from zero to seven, and wherein the subcarrier skipping parameterranges from zero to seven.
 19. A network unit in a coaxial networkcomprising: a receiver configured to receive a message indicating anassigned upstream training block in an upstream orthogonal frequencydivision multiplexing (OFDM) symbol comprising a plurality of pilotsubcarriers equally spaced in an upstream frequency spectrum of thenetwork unit in a pre-determined time interval, wherein the upstreamtraining block comprises a subset of the pilot subcarriers that arenon-consecutive and situated across the upstream frequency spectrum; aprocessor coupled to the receiver and configured to generate theupstream training block by modulating a pre-determined sequence onto thepilot subcarriers of the upstream training block; and a transmittercoupled to the processor and configured to send the upstream trainingblock via the coaxial network.
 20. The network unit of claim 19, whereinthe processor is further configured to generate the upstream trainingblock by setting subcarriers excluded from the subset of the pilotsubcarriers to values of zeroes, and wherein the message comprises: afirst pilot subcarrier position assigned to the upstream training blockin the upstream OFDM symbol; and a number of un-assigned pilotsubcarriers between successive assigned pilot subcarriers.